1. Field of the Invention
This invention is for a flat nuclear magnetic resonance coil, particularly a flat coil for examining thin samples of material(s), or for evaluating a thin region close to a surface of a sample. More specifically, the flat nuclear magnetic resonance coil according to this invention works well in a static magnetic field, for example Earth's magnetic field, that is oriented generally perpendicularly to the surface of the flat sample and plane of the coil.
2. Background Art
Nuclear magnetic resonance (“NMR”) is a well-established technique with applications in fields such as physics, chemistry, mechanical engineering, chemical engineering, petroleum engineering, biology, and medicine. Most applications of NMR use strong static magnetic fields from magnets (a wide variety of types of magnets are employed in different systems, depending upon the system), functioning together with electrical current-carrying coils that generate time-varying magnetic fields at samples, to receive NMR signals from the samples that are induced by the time-varying magnetic fields. In this disclosure, “sample” refers to any object or material that is the intended subject of examination by NMR process. The strong static magnetic fields usually are generated by magnets that surround the sample. Examples include cylindrical horizontal superconducting magnets used for medical magnetic resonance imaging (“MRI”). In medical MRI, the current-carrying coils normally reside within the cylindrical superconducting magnet, and a patient is placed inside the specially designed coils (typified by the so-called “birdcage” coils). Other examples include vertical superconducting magnets used for chemical analysis, in which small sample-containing glass tubes are lowered into coils situated in the bore of the vertical superconducting magnets. Yet other examples include permanent magnets that do not require superconductors (which currently require cryogens). Permanent magnets usually have their sample spaces defined by steel poles, north and south, between which the samples in coils are placed. In a minority of examples, the sample is placed to one side of the magnet and coil to allow the examination of samples too large to fit inside the magnets. This class of single-sided or remote NMR experiments includes large-scale experiments in which the magnetic field of the Earth is used in lieu of artificial magnets, and in which some type of coil is spread out on the ground surface to detect signals from underground samples such as, for instance, water in aquifers.
The most critical parameter for NMR process is the signal-to-noise ratio, S/N. This ratio increases steeply in stronger magnetic fields; consequently there is a major push in the NMR field of endeavor to perform NMR experiments and evaluations in the strongest possible magnetic field. However, the cost, size, weight, and care required rises with both the strength of the magnetic field and the size of the region in which the magnetic field is generated. Thus, there is a critical need to attain the optimal S/N ratio at almost any field strength, but especially at the weaker magnetic field strengths that are, by necessity, used for the larger samples. The NMR coil represents the most critical component of the NMR apparatus for defining the maximum S/N available from a given sample.
Conventionally, the common NMR sample is not extremely “flat”—that is, its aspect ratio of any two orthogonal dimensions is fairly close to one. As examples, consider the human body for medical MRI, or a test tube-like geometry used in analytical chemistry, in which the large dimension generally is, at most, about ten times the small dimension. These aspect ratios are consistent with commonly used coil geometry, such as a solenoid with an aspect ratio close to unity. When the sample that requires inspection is flat, i.e., some of the aspect ratios being larger than, say, 100, the traditional coil geometries (such as a solenoid) are inefficient due to the mismatch between the geometries of the sample and the coil.
By way of further illustration, consider the sample to be a flat piece of paper and the coil available is a solenoid. Unless the paper can be rolled up, it will occupy an infinitesimal volume inside a hypothetical solenoid that is large enough to accommodate the extent of the paper. Another example is a case where signal is to be obtained from only a thin layer near the surface of an object such as the skin of an animal. Thus, there is a need for NMR coils that are tailored for flat high aspect ratio samples. This requirement is true for cases such as Earth's field NMR measurement of moisture content in soil at fairly shallow depths. The use of Earth's field avoids the need to use heavy and expensive magnets, but its relative weakness requires that a large amount of sample be examined. Thus, the geometry for such a situation is a flat coil that is large enough to generate a sufficient S/N ratio for practical use.
There are examples of prior art in which a coil has been designed to be sensitive to NMR signals from a relatively flat region of an object that was located to one side of the coil and experiencing a static magnetic field. In all such cases, and in contrast with the system and apparatus of the present disclosure, the sensitivity profile has not been uniform at a given distance from the coil; as an undesirable result, under examination a thin sample yielded much less than optimal signal-to-noise ratio S/N.
A loop of wire forming a “surface coil” used to obtain NMR signals from the neighborhood of the coil that is placed on the surface of a sample, as opposed to deep inside a three dimensional sample, has been proposed by Ackerman. J. J. H. Ackerman, et al., “Mapping of metabolites in whole animals by 31P NMR using surface coils,” Nature 283, 167-170 (1980). When such a coil is placed close to the region of interest, the S/N ratio can be much better than a volume coil that is not tailored to the specific geometry. However, the simple loop of Ackerman et al. is not well-suited for use with a sample having a large aspect ratio (i.e., that which is much larger in horizontal extent compared to its thickness), because the sensitivity is not uniform in a plane close to the loop. Further, it does not work well in a static magnetic field that lies perpendicular to the plane of the coil—as opposed to the present invention, which works well in that orientation as well as parallel to the plane and to the wires simultaneously.
Array coils have been designed to receive NMR signals from a multitude of overlapping and closely positioned surface coils that can cover the region of interest, as described in P. B. Roemer, et al., “The NMR Phased Array,” Magn. Reson. Med. 16 , 192-225 (1990). Such coils are used in medical MRI, for example, to study the spine. An array of surface coils produces separate but overlapping data that are combined numerically to produce NMR signals. The aspect ratio (extent to depth) of the sampled space depends on the ratio of the overall size of the array to the size of the individual coil. Therefore, the number of coil elements, as well as the numerical complexity required, increases undesirably rapidly for large aspect ratios. And once again, the optimal static field direction is parallel to the plane of the coil array.
A “meanderline coil” is a surface coil with currents in an array of parallel wires with alternating directions; that is, the current flow in adjacent parallel wires is in opposite directions. Meanderline coils specifically designed for NMR of flat samples in biology have been described. T. Nakada, et al., “31P NMR Spectroscopy of the Stomach by Zig-Zag Coil,” Magn. Reson. Med. 5, 449-455 (1987). They also have been utilized for studies of multiple-layer materials and coatings, where signals from a shallow sample region are desired without signals from the deeper regions of the sample that represent interference. Such a use of meander line coils is shown in, for example, U.S. Pat. No. 6,326,787 to Cowgill, the contents of which are incorporated herein by reference by way of useful background. These applications are useful in static magnetic fields that lie parallel to the wires (i.e., in a specific direction in the plane of the coil), but are not useful for static field components out of the plane of the coil. In the case, for example, of the Earth's field NMR, where the static field is mostly out of the plane of the coil, the component of the coil-generated magnetic field that is perpendicular to the static field is extremely non-uniform near the coil. The specific component oscillates in space because the current directions are opposite in adjacent wires. The resulting non-uniform sensitivity, at a given depth, makes the S/N quite poor for a sample close to the coil. And thin samples to be evaluated typically are placed close to the coil. (In contrast, the invention disclosed hereinafter will work well, not only with the field normal to the plane of the coil, but also for the case suited for meanderline, that is, with the coil wires parallel to the static field). The meander line coil has also been used in nuclear quadrupole resonance of solids, where the alternating direction of the currents in adjacent wires do not matter because there is no static magnetic field. M. L. Buess, et al., “NQR Detection Using a Meanderline Surface Coil,” J. Magn. Reson. 92, 348-362 (1991). In sum, the meanderline type of coil does not work well for static magnetic fields that are situated substantially perpendicular to the plane of the coil, because the component of the RF field generated by the coil that is orthogonal to the static field is very non-uniform. This is especially true in regions that are halfway between adjacent wire segments, where the predominant component of the generated field is parallel to the static field.
Surface GARFIELD, as described by P. J. McDonald, et al., “A unilateral NMR magnet for sub-structure analysis in the built environment: The Surface GARField,” Journal of Magnetic Resonance, 185, 1-11 (2007), is a NMR system containing a specialized magnet that is designed to obtain diffusion measurements, not NMR intensity measurements, in a thin region near the surface of a material. The special array coil is designed to work only with the specific arrangement of the permanent magnets that generate magnetic field gradients, and not with a uniform static magnetic field from an external magnet including the Earth's magnetic field.
A transmission line coil has been designed specifically to have an extremely low-Q for rapid recovery of the NMR system for the study of planar solids with very short spin-spin relaxation time T2. Lowe, I. J., and Engelsberg, M., “A fast recovery pulsed nuclear magnetic resonance sample probe using a delay line,” Review of Scientific Instruments 45, 631-639 (1974). Similarly, there have been applications in membrane biology in which thin samples need to be examined, and for which specialized coils have been designed, such as those suggested in N. C. Nielsen, et al., “A flat-coil NMR probe with hydration control of oriented phospholipid bilayer samples,” Journal of Biomolecular NMR, 5 (3), 311-314 (1995). In both of the immediately foregoing cases, the coil encloses the sample and, therefore, is not a surface coil that is suitable solely for separated thin samples (and not samples that are a part of a larger object).
STRAFI is the name of an NMR technique whereby the sample is placed in the stray magnetic field away from the center of the magnet, often even outside the magnet, and a very thin region is examined because of the steep magnetic field gradient that exists at the sample. STRAFI is described in P. J. McDonald and B. Newling, “Stray field magnetic resonance imaging,” Rep. Prog. Phys. 61, 1441-1493 (1998). Because NMR “looks” for signals only at a certain frequency, i.e., the Larmor frequency, the steep magnetic field gradient insures that the signal arises from only a very thin region having the requisite Larmor frequency. In the STRAFI measurements, no special coil is used to optimize the S/N ratio. Instead, a solenoid is usually used to enclose a sample that is not especially flat, and a flat “slice” of the sample is rendered NMR-sensitive by the steep magnetic field gradient. Thus, the flat sample region accessible to STRAFI evaluation techniques is not due to any special coils, but rather to the way the static magnetic field is formed.
Against the foregoing background, the presently disclosed apparatus and method were developed. The present apparatus is of a coil that is tailored specifically to examine flat samples located to one side of the coil, and is especially suited to, but not limited to, single-sided NMR. The method of the presently disclosed invention is especially attractive because, as shall be described later herein, it performs well with a static magnetic field perpendicular to the plane of the coil.